The present invention relates to a spot-type disc brake, primarily for automotive vehicles. Brake shoes are arranged on either side of the brake disc. A brake support member receiving the brake torque supports the guide rods on which a brake caliper is guided slidably on the brake support member straddling the brake disc. The brake caliper is provided with an actuating device to urge the brake shoes against the brake disc. The brake caliper is resiliently biased in the brake release direction by means of spring whose force is maintained constant independently of the pad thickness. A friction element that is slidably supported on the guide rod relative to the brake support member is positioned to maintain the spring force constant.
A spot-type disc brake of this kind is known from the German printed and published patent application No. 24 14 174. Friction discs are used in the known device which serve as braces for a spring force applied to the brake caliper. When on account of the degree of wear of the pad, the slide of the brake caliper exceeds the distance between the friction disc and a stop in the release position of the brake, the friction disc is shifted so that the amount of spring force applied to the brake caliper is constant at all times. On releasing the brake, the brake caliper is forced back for a determined length in the direction of the brake release to ensure that the brake shoe travels back and away from the brake disc by a determined brake clearance after each normal braking action.
In the event of braking actions with extremely high actuating forces, the brake caliper, too, is as a rule subject to elastic deformation. This kind of expansion of the brake caliper on so-called emergency braking operations may lead to the friction disc being shifted back excessively since the elastic deformation of the brake caliper simulates a wear of the brake pad which has not really occured. It may therefore well be that after an emergency braking, the brake caliper although being forced back by the amount of the brake clearance, nevertheless abuts against the brake disc after the brake has been released. The disc brake will in this case be restored to normal condition only when the pad thickness has decreased by the brake clearance. However, this abrasive action causes an undesirable heating of the brake device. Moreover, the brake pad is worn to an extent greater than necessary for proper braking. In addition, brake action of this type obviously leads to an alteration of the vehicle behaviour especially in terms of the acceleration of characeteristics. Furthermore, the work performed during the abrasive action has to be balanced additionally by the driving engine.